The present invention relates to image-forming optical systems and viewing optical systems. More particularly, the present invention relates to decentered optical systems with a reflecting surface having a power for use in optical apparatus using a small-sized image pickup device, e.g. video cameras, digital still cameras, film scanners, and endoscopes, and also in optical apparatus using a small-sized image display device, e.g. head-mounted image display apparatus.
In recent years, head- or face-mounted image display apparatus have been developed for the purpose of enabling the user to enjoy a wide-screen image personally.
Japanese Patent Application Unexamined Publication (KOKAI) Number [hereinafter referred to as "JP(A)"] 7-333551 proposes an ocular optical system that leads an image displayed on an image display device to an observer's eyeball. The ocular optical system is formed from a decentered prism optical system having three optical surfaces that face each other across a medium having a refractive index larger than 1. A light beam from a liquid crystal display device is made to enter the decentered prism optical system through the third surface thereof. Then, the light beam is totally reflected in the decentered prism optical system by the first surface thereof. The reflected light beam is internally reflected by the second surface, which is a concave mirror, so as to exit from the decentered prism optical system through the first surface, which serves as both reflecting and transmitting surfaces, thereby leading the image displayed on the image display device to an observer's eyeball without forming an intermediate image.
In this case, the number of optical surfaces constituting the decentered prism optical system is three, and the number of reflections in the decentered prism optical system is two. The present applicant et al. have proposed various types of decentered prism optical systems in which the number of constituent optical surfaces is two or four or more and the number of reflections taking place in the optical system is one or more. The present applicant has also proposed using such a decentered prism optical system as an image-forming optical system, e.g. an objective optical system of a camera.
JP(A) 10-153748 proposes an ocular optical system in which a relay lens system and a decentered prism optical system are combined together so that an image displayed on an image display device is led to an observer's eyeball after an intermediate image has been formed once by the relay lens system. The decentered prism optical system has three optical surfaces, and three reflections take place in the decentered prism optical system.
Incidentally, when an image display device used in an image display apparatus is compact in size, the viewing field angle cannot be widened unless the focal length of the ocular optical system is shortened. However, when the focal length of the ocular optical system is shortened, the exit pupil position comes close to the optical system. Therefore, it has heretofore been impossible to construct an ocular optical system having a short focal length while ensuring the required eye point distance.
In JP(A) 10-153748, a rotationally symmetric optical system is used in a relay optical system that forms an intermediate image. However, the ocular optical system in JP(A) 10-153748 uses a decentered prism optical system in an optical system that projects the intermediate image at a distant place. Therefore, decentration aberrations are produced by the decentered prism. In particular, when the image display device has a high display density, the ocular optical system is required to have optical performance that provides very high resolution. However, it is very difficult to reduce aberrations due to decentration, particularly the tilt of a first image-formation plane with respect to the principal ray, rotationally asymmetric curvature of field and rotationally asymmetric astigmatism. Such decentration aberrations cannot be corrected simply by tilting a rotationally symmetric relay optical system as in JP(A) 10-153748. The decentration aberrations can be corrected to a certain extent by using a free-form surface. However, it is impossible to correct decentration aberrations completely.